Field of the Invention
The invention relates to a method for operating a nuclear-magnetic flowmeter. The nuclear-magnetic flowmeter thereby has a measuring tube with at least one first measuring section for carrying out the method. In the method, a medium is made to flow through the measuring tube and the medium in the measuring tube is magnetized.
Description of Related Art
The invention also relates to nuclear-magnetic flowmeters. The nuclear-magnetic flowmeters have a measuring tube, a magnetization means and a measuring means, wherein the measuring tube has at least one first measuring section. During operation of the nuclear-magnetic flowmeter, a medium flows through the measuring tube and the magnetization means magnetizes the medium in the measuring tube.
A nuclear-magnetic flowmeter uses nuclear-magnetic resonance methods for analyzing and, in particular, for measuring the flow of a medium through the measuring tube. Nuclear-magnetic resonance methods influence the precession of atomic nuclei of a medium in the presence of a macroscopic magnetic field, which has been previously magnetized, by exciting the atomic nuclei by means of an electromagnetic pulse affecting the medium and evaluating the effect of the excitation on the atomic nuclei. A nuclear-magnetic flowmeter thus has a magnetization means for generating a magnetic field in a medium flowing through the measuring tube and has a measuring means for exciting the medium in the measuring tube and for measuring the effect of the excitation on the medium in the measuring tube. The measuring means is thereby also designed to carry out the method for operating a nuclear-magnetic flowmeter.
The atomic nuclei of the elements having a nuclear spin also have a magnetic moment caused by the nuclear spin. Nuclear spin can be regarded as an angular momentum describable by a vector, and accordingly, the magnetic moment can also be described by a vector that is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field, such as the one generated by the magnetization means, at the location of the atomic nucleus. The frequency of precession is called the Larmor frequency ωL and is proportional to the value of the magnetic field strength B. The Larmor frequency is calculated according to ωL=γ·B, wherein γ is the gyromagnetic ratio, which is at a maximum for hydrogen atom nuclei.
Nuclear-magnetic flowmeters measure the flow of a medium through the measuring tube. The flow of the medium relates either to a volume flow or to a mass flow, wherein both the volume flow and the mass flow are determined, in each case, using a density of the medium and a velocity of the medium in the measuring tube. Methods known from the prior art for operating a nuclear-magnetic flowmeter and known nuclear-magnetic flowmeters determine a velocity of a medium in a measuring tube in that a volume of the magnetized medium within a measuring section are excited to nuclear magnetic resonances and a signal sequence is formed. The signal sequence is thereby formed in that signals are determined that characterize the nuclear-magnetic resonances of the medium in volumes within the measuring section. The signals in the signal sequence subside over time, wherein the subsiding of the signals is influenced, in particular, by two factors. The first factor describes the subsiding of the signals over time essentially based on interaction of the nuclear spin of the atomic nuclei of the medium. This factor is dependent on the properties and the state of the medium, wherein the state, in particular, is described by the temperature of the medium. The second factor describes the subsiding of the signals over time based on the excited medium flowing out of the measuring section. This factor thus includes information about the velocity of the medium, whereas the first factor compromises this information.